This invention relates to two-part epoxy adhesives which have improved flexibility and a process for making and using them. More particularly, this invention relates to an improved epoxy adhesive which has good adhesion to various substrates, especially where a high level of elongation is needed due to thermal expansion and contraction of the substrates or where the substrates are exposed to thermal shock. It can also be used at low operating temperatures.
Polymeric compositions based on the commercially available Bisphenol A based epoxy resins cured by one or more of the active hydrogen compounds, such as polyamines, polyacids, polyphenols, and the like, exhibit good adhesion properties towards various substrates including metal and plastics, but most of these adhesives are rigid, stiff and brittle polymers. There is also a problem of shrinkage occurring in the epoxide adhesive during the curing, thus building undesired stress in the substrates, as well as in the adhesive at the glue line. Accordingly, it has been suggested that flexibilizers such as butadiene rubbers, urethane elastomers, and the like be added to the epoxy resins.
Past attempts to modify epoxies involved reacting them with copolymers of butadiene, acrylonitrile or other “rubber”-based compositions. See Journal of Applied Polymer Science, Vol. 26, pp. 907-919 (1981). By reacting an epoxy with those compounds and then processing the modified composition to induce in situ phase separation, “rubber rich” domains within the epoxy composition are created. Those domains improve the toughness of the cured epoxy matrix once it is formed. However, while such modified epoxy compositions are tougher and less brittle at room temperature, improvements at lower temperatures prove to be, at best, marginal. Further, using the above-mentioned rubber-based material incurs processing problems due to an increase in viscosity of the adhesive mixture.
Accordingly, various epoxy adhesive formulations have been proposed to address these problems. See, for example U.S. Pat. Nos. 4,728,384; 5,098,505; 5,218,063; 5,367,006 and 5,679,730. More recently, there has been introduced Mr. Sticky's Underwater Glue. According to the website at underwaterglue.com, this is an improved two-part epoxy glue having a flexibility (elongation) of 30%, very good shock dampening, and flexible bonds. It has a mixture working life of 30-60 minutes and an initial curing time of 3-4 hours. The complete curing time is 24 hours.
Still, there is a need for an even more improved two-part epoxy adhesive.